Apparatus for longitudinal orientation of thermoplastic film material

ABSTRACT

Apparatus for longitudinal orientation of thermoplastic film material ( 4 ) comprises a width-reduction zone upstream of the longitudinal stretching zone, through which the width of the film is gradually reduced so as to allow longitudinal stretching without necking. The width-reduction zone comprises at least one, preferably several, pairs of pleating rollers ( 16, 17 ) comprising intermeshing grooves or discs for pleating the material, the length of said width reduction zone preferably being less than 3 times the original width of the film. The width-reducing zone has upstream ( 14 ) and downstream ( 15 ) rollers or roller assemblies, with curved axes and optionally also conveyor belts for providing smooth width reduction. Longitudinal stretching is preferably between at least two, and preferably more than two, pairs of stretching rollers ( 9, 10, 11, 12 ) which are relatively close together. The pleats may be provided in several stages with increasing pleats per stage, while guiding means preferably lay all the pleats to one side. The apparatus allows achievement of high tensile strength, yield point, resistance to tear propagation and puncture resistance, especially for polyethylene and polypropylene films.

RELATED APPLICATIONS

This application is a divisional application of and claims the benefitof and priority to U.S. patent application Ser. No. 12/740,203 filed 1Dec. 2010, now U.S. Pat. No. 8,784,716 issued 22 Jul. 2014, which claimsthe benefit of and priority to PCT/EP2008/064735 filed 30 Oct. 2008,published as WO2009056601 published on 7 May 2009, which claims priorityto GB0721410.9, filed 31 Oct. 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns method and apparatus for longitudinal orientationof thermoplastic film material, especially with a view to themanufacture of crosslaminates of films which have been uniaxiallystretched at a relatively low temperature. It is known that the bestall-round strength properties in a crosslaminate are obtained by thefollowing steps of orientation: first a strong almost uniaxialmelt-orientation during the drawdown from the extrusion die, or stillbetter an almost uniaxial orientation while the polymer material issemimolten, and then further orientation at a rather low temperature.“All-round strength properties” here refers to a combination of tensilestrength, yield point, tear propagation resistance and punctureresistance. It is difficult to give a satisfactory explanation why thiscombination of orientation steps is preferable, but it can briefly besaid that when the orientation is carried out in these steps, themolecular chains will exhibit a wide spectrum of different degrees oforientation, and those of relatively low orientation will help the filmto re-orient instead of splitting, when it is subjected to tearing orpuncturing forces.

However, the stretching at low temperature causes significant problems,e.g. in films which may consist of high density polyethylene (HDPE) orisotactic or syndiotactic polypropylene (PP). One side of this problemis that, when a film is longitudinally stretched, it has a high tendencyto contract in the transverse direction, at the same time as itsthickness is reduced. This tendency is highest when the temperature islow, e.g. between 10-40° C. which is optimum stretching temperaturerange for HDPE and PP, as far as the achieved properties are concerned.The other side of the problem is that, at these low temperatures thematerial tends to “neck in”, instead of gradually developing theorientation within a reasonably long zone. This means that thestretching must take place between closely spaced stretching rollers orstretching bars, and unless special precautions are taken this willprevent the film from undergoing the needed contraction in thetransverse direction.

2. Description of the Related Art

In the inventor's patent U.S. Pat. No. 3,233,029, which was publishedabout 40 years ago, a proposal is made for solution of this problem,namely, to “anticipate” a substantial part of the transverse contractionto which the film tends by longitudinal pleating prior to a stretchingwithin one or more short stretching zones, as this is more exactlyexpressed as follows: the stretching taking place in one or more shortzones between and/or on two or more stretching rollers or bars, in whichprocess the width measured in a straight line from edge to edge isreduced prior to the stretching, this reduction being in form of aregular pattern of longitudinally extending pleats, whereby thereduction of width and the length of the stretching zones are adapted toallow the film a full straightening out of the pleats by the inherenttendency in the polymer material to contract transversely while beingstretched longitudinally.

In that patent, the pleating mechanism described consists of two sets ofdiscs which are mounted spaced apart upon the shafts, one over and oneunder the film to become pleated, so that discs in one set mesh betweenthe discs in the other set. Thereby the film is forced to form folds orconvolutions. It is further disclosed that the film preferably shallpass over a crown-shaped roller adapted to make the stress upon theborders equal to that in the middle of the film. Crown-shaping meansthat the roller has highest diameter in its middle, the diametergradually decreasing towards its ends. Finally it is disclosed that thefilm preferably is cooled in the stretching zone, which may convenientlybe by covering a stretching bar with felt and keeping this felt wet. Thewater also, by its lubricating action, helps to allow the film thetransverse contraction which eliminates the pleats. No pleats remain inthe final product.

The inventor managed to make this old invention work with flexibilizedHDPE and PP, but only in relatively narrow widths, insufficient for anindustrial production e.g. of cross-laminated industrial bags orcross-laminated coversheet. When trying to apply the invention tostiffer film, such as film made from plain HDPE or PP, or when trying iton film of greater width, e.g. 1 m wide, the transverse forces appliedby the film always caused a transverse stretching of the film in theform of thin, longitudinally extending lines. It appears that theprinciple to apply longitudinal pleating thereby allowing a filmtransverse contraction during longitudinal stretching, has up to nowonly been carried out industrially under conditions which also producetransverse stretching and attenuation along narrow longitudinal lines.

SUMMARY OF THE INVENTION

The present invention provides an improved process of supplying athermoplastic polymer film with a longitudinal orientation by stretchingin solid state. The stretching taking place in one or more short zonesbetween and/or on two or more stretching rollers or bars, in whichprocess the width measured in a straight line from edge to edge isreduced prior to the stretching. This reduction being in form of aregular pattern of longitudinally extending pleats, whereby thereduction of width and the length of the stretching zones are adapted toallow the film a full straightening out of the pleats by the inherenttendency in the polymer material to contract transversely while beingstretched longitudinally, and whereby the formation of said pleats takesplace between at least one pair of mutually intermeshing grooved rollersor intermeshing sets of discs. The improvement being characterized inthat the reduction of width takes place gradually within a reductionzone no shorter than half of the original film width, this zone beinglimited by an upstream roller or roller assembly and a downstream rolleror roller assembly installed with varying directions of the axis ofrotation, this direction forming an angle of 90° with the machinedirection at the middle of the film and gradually changing towards itsedges to forward the film in converging manner within the reductionzone.

The present invention provides an apparatus for longitudinal orientationof a thermoplastic film (4) comprising, in sequence in the machinedirection, i) a width-reducing station comprising at least one pair ofintermeshing pleating rollers (16) comprising intermeshing groovedrollers or intermeshing sets of discs for application of regularlongitudinally extending pleats across the width of the film; and ii) alongitudinal stretching station for stretching the film longitudinallyin the solid state, comprising one or more spaced pairs of stretchingrollers (9, 10) or bars, the spacing between each said pair beingrelatively short, characterized in that the width-reducing stationcomprises an upstream roller (14) or roller assembly and a downstreamroller (15) or roller assembly with a width reduction zone locatedtherebetween, the upstream and downstream roller or roller assemblyhaving varying directions for the respective axis of rotation, thevariation being from 90° with the machine direction at the center of thefilm, gradually varying on both sides of the center towards the edges ofthe film, so that the film is directed through the width reducingstation while the edges of the film converge towards the center.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention shall now be described in further detail with reference tothe figures.

FIG. 1 is a photograph illustrating, in perspective view the basicprinciple that the tendency to transverse contraction of a film duringlongitudinal orientation is allowed to take place by feeding the filminto the stretching zone in pleated state. The photo shows a sample of alay-flat tubular film, mainly consisting of HDPE, before and after thestretching described in Example 1. The sample was taken of out thecold-stretching machine during a stop.

FIGS. 2 a and 2 b are principal sketches showing a line for m.d.stretching, including the devices for pleating the film prior to thestretching. FIG. 2 a, which represents the entire line, is a verticalsection comprising the section along a-a in

FIG. 2 b. FIG. 2 b is the horizontal section along b-b in FIG. 2 a. Forthe sake of clear illustration the distance between the differentrollers are generally shown disproportionately short in comparison withthe diameters of the rollers. Also the grooved surface patterns of thepleating rollers are not shown.

FIG. 3 shows a diagrammatic section through the axes a segment of a pairof intermeshing grooved rollers forming, adjusting or controlling thepleating. In case the grooved rollers are banana rollers, it should beunderstood as a folded-out view.

FIGS. 4 a and b are geometrical drawings serving as basis forcalculations of parameters for the pleating process.

FIG. 5 shows in detail a preferred technical construction of a groovedbanana roller in principle like FIG. 3, but in a more solid and durableconstruction.

FIG. 6 shows in detail a construction of the middle of a banana rollerat the inlet to or exit from the width reduction zone.

FIG. 7 shows, in schematic perspective view, several positions of apleated film, as the pleats are laid flat on a smooth roller. For thesake of clarity the guiding tracks and the smooth roller are not shown.

FIG. 8 shows an alternative to the banana roller of FIG. 6, namely anarray of short smooth rollers forming part of a polygon, which approachan arc of a circle.

FIG. 9 shows a similar alternative to the grooved banana roller of FIGS.3 and 5, namely an array of short grooved rollers forming part of apolygon, which approaches an arc of a circle.

DETAILED DESCRIPTION OF THE INVENTION

The described problem is overcome by the improvement: characterized inthat the reduction of width takes place gradually within a reductionzone no shorter than half of the original film width, this zone beinglimited by an upstream roller or roller assembly and a downstream rolleror roller assembly installed with varying directions of the axis ofrotation, this direction forming an angle of 90° with the machinedirection at the middle of the film and gradually changing towards itsedges to forward the film in converging manner within the reductionzone. By forwarding the film in the mentioned converging manner, thetransversely acting forces exerted by the pleating devices are reducedand can be almost entirely eliminated by optimized adjustments, so thatthe formation of transversely stretched, attenuated longitudinal linesis avoided. The preferable degree of pleating, i.e. the ratio betweenthe film width before pleating and after pleating, the latter measuredalong a straight line from edge to edge, will be discussed in thespecific description.

The invention has particular importance in connection with longitudinalstretching of the film which mainly consists of HDPE, PP or blends ofthese polymers, since the relatively low price and their stiffness andachievable strength properties make them most suitable forcrosslaminates used in industrial articles such as e.g. industrial bags,cover sheets, tarpaulins, reinforced bitumen sheets, pond liners,greenhouse film and “house-wrap-film”. However, the invention is alsoapplicable to all other film of thermoplastic polymer material, if suchfilm in the form of narrow strips can be oriented at or near normal roomtemperature. As examples the invention is applicable to film based onpolyamides, polyesters such as polyethyleneterephthalate,polyvinylidenechloride and crystalline copolymers of vinylchloride andvinylindenechloride. It is also expected to become useful for filmsbased on cold-stretchable biodegradable polymer materials.

Most conveniently, the reduction zone should be no longer than 3 timesthe original film width, preferably no more than twice and still morepreferably no more than equal to the width.

The advantages of stretching at relatively low temperatures have beenmentioned above, and the stretching according to the invention shouldnormally take place at a temperature no higher than 60° C., preferablyno higher than 50° C., and still more preferably no more than 40° C. Thefilm to be stretched may be in the form of a lay-flat tube. This has inparticular reference to the manufacture of crosslaminates fromuniaxially oriented films, since the normal manufacturing process forsuch crosslaminates comprises a step of helical cutting of alongitudinally oriented tube.

In a preferred embodiment of the invention the downstream roller orroller assembly comprises at least one banana roller with its convexside pointing towards the upstream roller assembly. “Banana roller” isthe name normally used for a roller having a curved axis, usually formedas an arc of a circle. Banana rollers are normally used to removewrinkles or pleats, but here used to the contrary. In its simplest forma banana roller consists of a slightly bent shaft put into a rubberpipe, which can rotate on this shaft. The pipe is normally lubricatede.g. with talc. In a more industrial design, there is an array of ballbearings or roller bearings, closely side by side between the bent shaftand the rubber pipe. The rubber pipe can e.g. be substituted by an arrayof rings, each one fitted to a bearing.

The bending of the banana roller may be adjustable. Adjustment of aconventional banana roller is well known and can take place byadjustment of the angular position of the ends of the shaft. To allow avariable bending, the shaft is preferably made from a composite, e.g. ofglass fibres or carbon fibres embedded in polymer material.

No matter whether a banana roller with adjustable or fixed bending isused, the radius of the bending is determined by the length of thecontraction zone and the chosen degree of pleating. This is explained inthe specific description.

As a technical equivalent to the use of at least one banana roller asdownstream roller, or as part of the downstream roller assembly, theremy be provided many short rollers mounted individually in such a waythat together they form part of a polygon approaching an arc of acircle.

While the film leaves the last roller or roller assembly of thedownstream part of the reduction zone and proceeds towards thestretching rollers or bars, it is preferably guided in a directionsubstantially perpendicular to its movement with the reduction zone,preferably deviating no more than 10° from being perpendicular to thisdirection. As it shall be further explained in connection with thedescription of the drawings, this precaution serves to equalize thelongitudinal tensions in the film over its width.

Similar to the construction of the downstream roller or roller assembly,the upstream roller or roller assembly may conveniently consist of abanana roller or several parallel banana rollers with the concave sideor sides pointing towards the downstream roller or roller assembly. Saidbanana roller or rollers preferably form arcs, the tangents of which atany location are perpendicular to the film tension created by thedownstream roller or roller assembly. This means, if upstream anddownstream rollers are banana rollers, that these rollers form generallyconcentric arcs. This is further clarified in the specific descriptionof the preferred embodiment.

The upstream banana roller or the last one of the upstream bananarollers can with advantage be supplied with an array of protrudingcircular segment parts to start the pleating. Also similar to theconstruction downstream of the reduction zone, the bending of eachbanana roller can be made adjustable.

As already mentioned, the film preferably exits from the downstream partof the reduction zone substantially perpendicular to its movement withinthis zone. It also preferably and for similar reasons is forwardedtowards the first upstream roller in a direction substantiallyperpendicular to its movement within the reduction zone, preferablydeviating at the highest 10° from this direction.

s an alternative to the use of one or more banana rollers at the inletto the reduction zone the upstream roller or roller assembly can be acrown-shaped roller or roller assembly of short rollers which togetherform a crown-shape on a straight shaft, said short rollers beingconnected with a common shaft through bearings to be rotatableindependently of each other.

The gradual reduction of width within the reduction zone is preferablyassisted by grooved banana rollers installed between the upstream rolleror roller assembly and the downstream roller or roller assembly, or inother manner by arrays or mutually intermeshing discs. A grooved bananaroller for this purpose may consist of discs of different externaldiameters in alternating succession or of short roller segments suppliedwith grooves, whereby the discs or roller segments are mounted on a bentshaft through bearings, or they may in themselves act as bearings.

Preferably the intermeshing between two such grooved banana rollers isnot set in a fixed manner, but is made variable by means of anadjustable force trying to push the two grooved banana rollers together.This adjustable force may be created by springs, pneumatic means or bygravity. When the pleating is irregular, as generally at the start-up,the forces which try to increase the intermeshing will act most stronglyon the film where the degree of pleating is lowest. Provided the meansto push the two grooved banana rollers together has been properlyadjusted by experimentation, the pleating gradually will become even allover the width of the film.

If the intermeshing between the grooved banana rollers is set in a fixedmanner without special precautions being taken, or if the adjustableforce trying to push the rollers together is set too high, the resultmay be that, instead of gradually making the pleating even, the groovedbanana rollers perform transverse stretching of the film where thedegree of pleating is lowest, hereby creating longitudinally extendingthin lines.

It has been found that the system of pleating in an evenly pleated filmunder zero or low tension tend to become randomized when the film passesover a smooth banana roller or a smooth or straight roller. This can bea problem in connection with the smooth rollers used in the presentinvention before the film has become permanently elongated. Tocounteract this randomization there may be guiding means actingimmediately upstream of and in close proximity to such smooth roller.These means may preferably be tracks adapted to fold all pleats over tothe same side.

The pleating as described is preferably carried out in several stepswith several sets of intermeshing grooved banana rollers or arrays ofdiscs, the pitch of the arrays in said sets developing from a coarser toa finer pleating.

Alternatively to the described use of grooved banana rollers, thegradual reduction of width within the reduction zone may be assisted bya set of narrow conveyor belts following and guiding the film through atleast a part of this zone, such that the two sets of narrow beltsgradually intermesh more and more with each other during the convergingadvancement in the zone.

The invention also concerns any apparatus suitable for carrying out themethod described above, and it is emphasized that banana rollers withgrooves suitable for forming or controlling pleating in themselves isconsidered an invention.

In the photograph FIG. 1, the zone indicated as (1) is the pleated HDPEfilm before any orientation has been carried out. The zone (2) has beenthrough a first step of stretching at 15° C., namely in the ratio 1.5:1,by which it has become m.d.-oriented within “stretching lines” extendingon the bias and crisscrossing each other. The majority of the film isstill not oriented, apart from its melt orientation. The orientationproduced by the cold stretching can directly be observed, sincestretching of film from HDPE or PP at temperatures lower than about40-50° C. creates closed micro-voids, which act like grains of whitepigment. This is well known.

The zone (3) has been through a 3rd step of stretching, in example (1)the final step, and has turned white all over, while the biased,criss-crossing stretching lines gradually have grown and developed intoa structure which on macro scale is homogeneous. At the same time thefilm has contracted in the transverse direction, and the pleats havedisappeared. The final stretch ratio before relaxation was 3.8:1 andafter relaxation 2.8:1.

The described development of the orientation process, starting withbiased, mutually criss-crossing “stretching lines” and continuing as agradual developing and growing together of these “lines” means that thefinal oriented film become inhomogeneous seen on a micro scale. Therewill be micro-regions all over the film having zero or almost zeroorientation (apart from the melt orientation), and there will bemicro-regions all over the film in which the orientation forms a smallangle to the orientation in adjacent micro-regions. This sort ofmicro-inhomogeneities will help the film to re-orient when subjected totransverse forces, and is therefore very advantageous for the tearpropagation resistance and puncture resistance in a crosslaminate madefrom such stretched films.

As it appears from example 1, the pitch of each of the final groovedrollers for pleating has been 15 mm, and the average “wavelength” of thepleats corresponds hereto. The required fineness of the pleating dependson the transverse contraction forces during the cold stretching and thefriction between the film and the withholding stretching roller orrollers. Low transverse contraction forces and/or high friction requirea particularly fine pleating.

In FIGS. 2 a and b, the film (4) is taken from reel (5) and passedthrough nip rollers (6) which by means of automatic tension measurementand a brake system (not shown) serve to keep the tension at an adjustedvalue.

The film proceeds from roller pair (6) through a section, which isdiscussed in connection with FIG. 4 b, to a first smooth banana roller(14) acting as inlet to the width-reducing zone. The exit from this zoneis the smooth banana roller (15), and between the smooth banana rollersare installed three pairs of mutually intermeshing grooved bananarollers (16, 17 and 18). The plane determined by the circular axis ofthe smooth banana roller (14) is essentially identical with the planedetermined by the circular axis of the smooth banana roller (15),although small deviations are permissible, and similarly the planesdetermined by the circular axes of each of the grooved rollers in thewidth reduction zone, are close to following the same plane. All of thecircular axes of the banana rollers are essentially concentric. Thechoice of radii for these circular arcs is discussed in connection withFIG. 4 a.

As discussed in the general description, each of the three pairs ofgrooved banana rollers are constantly pressed together under an adjustedpressure, but this is not shown in the drawing.

While passing over the smooth banana roller (15), the film changesdirection and exits from this roller under an angle close toperpendicular to the direction it followed through the width reductionzone. On its way to the first roller in the stretching part of themachine, i.e., roller (7), it passes two pairs of mutually intermeshing,straight grooved rollers (19 and 20). These two pairs of grooved rollersare also constantly pressed together under an adjusted pressure (meansnot shown).

Banana rollers (14) and (15) and all grooved rollers, curved orstraight, are idle rollers.

Immediately upstream of each of the smooth rollers (15) and (7) and inclose proximity to each of these rollers, there are guiding means toavoid the smooth rollers randomizing the even pleating. For the sake ofclarity these guiding means are not shown here, but reference is made toFIG. 7 and the connected description.

The pleating of the film is primarily caused by the concentricarrangement of banana rollers (14) and (15) in combination with thetension in the film. However, these means alone will normally produce acoarse and uneven pleating.

In the described arrangement, the groove division on roller pair (16) isrelatively big, since it requires a relatively low force to form coarse,even pleats or convolutions, and since fine pleats formed at thisposition might tend to turn into coarse pleats while proceeding toroller pair (17).

The pitch on roller pair (17) is adapted to double the number of pleatsformed by the roller pair (16), and the pitch on roller pair (18) makesa further doubling. The smooth banana roller (15) lays the pleats orconvolutions flat as described in connection with FIG. 7. There can beguiding means to secure that this happens in an even manner. To avoidthe pleats growing coarser on the way from roller to roller, thedistances from roller pair (17) to roller pair (18) and from roller pair(18) to roller pair (15) are relatively short.

On the way to the intermeshing straight grooved rollers (19) the pleatsagain take the stand-up form. The pitch on roller (19) is adapted tobring the number of pleats back to the number formed by roller pair(17). This is so chosen since in spite of the described guiding tracksthe smooth banana roller (15) can cause some disorder in the arrangementof the lay-flat pleats, and reestablishment of the order then requires abigger pitch. The number of pleats or convolutions is again doubled bythe passage through roller pair (20), and then maintained by the smooth,driven roller (7) and the rubber coated nip roller (8).

The distance between rollers (15) and (19) is relatively long. Thereason for this choice is discussed in connection with FIG. 4 b. Thedistance between roller pair (20) and rollers (7) and (8) is short toavoid the fine pleats from growing coarser. (The drawing is not quite toscale on these points.)

The smooth, driven roller (7) and its rubber coated counter roller (8)hold back on the film during the stretching, assisted by the smoothroller (9), which is driven at essentially the same circumferentialvelocity as roller (7).

The smooth rollers (10), (11), and (12) are also driven. Roller (13) isa rubber coated nip roller. Roller (10) moves faster than roller (9), tocarry out a first stretching step, roller (11) moves faster than roller(10) to carry out a second stretching step and roller (12) moves fasterthan roller (11) to carry out a third stretching step. Each of therollers (7) to (12) are maintained at a constant temperature by means ofcirculating water. This temperature may be equal to, slightly lowerthan, or somewhat higher than normal room temperature. If stretching,e.g., at 30° C. or 40° C. is chosen the film must be preheated, and thisis most simply achieved by keeping the environment at such elevatedtemperatures.

From the described stretching line the film proceeds to an annealingstation, where it is heated, e.g. to 60-80° C., and is allowed to relax.This is conventional apparatus and in the drawing is symbolized by a box(114). It should be noted, however, that the film, while being allowedto relax, will grow wider and thereby will tend to re-shape a partlypleated form, unless this is avoided, e.g., by use of several bananarollers.

The tension in a film during the relaxation is set by the velocity ofrollers (115) and (116), the latter with the rubber-coated counterroller (117), and this tension is automatically controlled by a tensionmeasuring banana roller (118). Finally the film is reeled up on winder(119).

The primitive grooved idle rollers shown in FIG. 3 can as mentionedeither be straight or curved. Thus, e.g., the two axes (20) can beunderstood as folded-out from planes which are perpendicular to thepaper plane. The shape of the pleating is adjusted and made even bymeans of the rings (22), which are idling on the fixed shafts (23). Therings (24) keep rings (22) accurately spaced apart from each other. Therings (22) and (24) are made from a self-lubricating material, e.g.,Teflon.

During start-up of the pleating/stretching machine line, each pair ofgrooved rollers should be out of engagement with each other. Theintermeshing is gradually established, e.g., by pneumatic means, whilethe line runs with film as this is explained in the general description.A more stable construction of a grooved banana roller is shown in FIG.5, and a straight grooved roller for pleating can of course be maderotating all over, with bearings at the ends.

In the calculations below relating to FIG. 4 a, there is made theapproximation that the axes of the two smooth banana rollers, in thisdrawing (AB) and (CD) and in FIG. 2 a (14) and (15), respectively, isequal to the radii of their convex and concave shapes. Thisapproximation is permissible since the radii of their cross sectionsnormally will be less than 3 cm.

(A), (E), (G), (I) and (C) represent one edge of the film at differentsteps of the process, and (B), (F), (H), (J) and (D) represent the otheredge. The distance from (A) to (B), measured along the arc, is the widthof the film as it enters the “width reduction zone”, and the distancefrom (C) to (D), also measured along the arc, is the width of thepleated film as it exits from this zone, (P) is the center of theconcentrical axes of the 5 arcs.

The degree of pleating is the ratio between the width of the unpleated,unstretched film and the pleated film as this enters roller (7) (seeFIG. 2 a). Width of pleated film is here measured straightly from edgeto edge. This ratio essentially equals the ratio between arc-length (AB)divided by arc-length (CD) which in turn essentially equals radius (PA)divided by radius (PC).

When a film strip of a few cm width is longitudinally stretched in ration:1 at relatively low temperatures, it will normally tend to reduce itswidth and thickness almost equally, i.e., both in a ratio about √n:1,however somewhat dependent on its melt orientation. Thus, e.g., atstretch ratio 4:1 it normally reduces both width and thickness in ratioabout 2:1. for stretching wide film in ratio 4:1, which was found to beabout the most which HDPE or PP film can be stretched without risk ofbreak, when the stretching temperature is about 20° C., the degree ofpleating should therefore theoretically be about 2:1. The stretch ratio4:1 here refers to the state in which no relaxation has taken place andthe film still is under the highest tension occurring during thestretching. However in practice it is very difficult to form a perfectlyeven pleating, and in order to secure that no traces of pleats remainafter the stretching, a degree of pleating between 1.5:1 and 1.6:1 wasfound most adequate with the mentioned stretching ratio 4:1, andtemperature about 20° C.

In the drawing the radius (PA) is 1.5 times radius (PC) corresponding todegree of pleating 1.5:1. It is further shown, that the length of the“reduction zone” equals the width (arc CD) of the fully pleated film,which has been found quite adequate. The angle between the two filmedges (AC) and (BD) therefore is 0.5 radian=28.6°.

In the following it is further stipulated that the length of arc (CD)and of the reduction zone is 1.00 m, the length of bow (AB) therefore1.5 m. Radius (PA) then will be 3.00 m and radius (PC) 2.00 m. Arc (EF)can suitably be located mid between arc (AB) and bow (CD), and arc (GH)can suitably be located mid between (EF) and (CD). This means that theradius of arc (EF) is 2.50 m and the radius of arc (GH) is 2.25 m. Asmentioned in connection with FIG. 2 a, arc (IJ) should be very close toarc (CD). It is stipulated that its radius shall be 2.08 m.

The distance between the middle of arc (AB) and chord (AB) is 3 m×(1−cos0.25)=9.4 cm. As it has been mentioned in the description of FIG. 1, therequired fineness of the pleating depends on the transverse contractionforces and the friction between the film and the withholding stretchingroller or rollers. It was found that a pitch of 15 mm on roller (7)normally is suitable for HDPE or PP based tubular film, if its gaugedoes not essentially exceed 0.10 mm. With reference to the descriptionof FIG. 2 a, the number of pleats passing the smooth banana roller (15)corresponds to this pitch. Consequently, taking the different radii intoaccount, the pitch on roller pair (18) is 15×2.08+2.00=15.6 mm. Rollerpair (17) is constructed to produce half the amount of pleats, and itspitch will be: 30×2.25+2.00=33.75 mm. Finally, roller pair (16) isconstructed to produce a number of pleats which is half of this, and itspitch will be: 60×2.5+2=75 mm. If the pleating is prepared by slightlyand smoothly protruding circular segment parts on grooved roller (14),the pitch of these protrusions will be: 60×3/2=90 mm.

The geometrical drawing FIG. 4 b is drawn in a plane perpendicular tothe plane of FIG. 4 a and going through the points which in FIG. 4 a arecalled (K) and (L). (M) is the point where the film leaves roller pair(6)—see FIG. 2 b. (N) on line (ML) is drawn such that (MK)=(MN).

(MK) is the route which the middle of the film passes from roller pair(6) to banana roller (14), and (ML) is the route which the film edgespass between the same rollers. Thus (LM) is the difference between thesetwo routes, and this difference creates differences in tensions. It isstipulated that a 1% difference is permissible, and the purpose of thefollowing calculations is to establish the minimum length of thedistance (KM).

The angle (LKN) is a periphery angle, thus half as big as angle (KMN),and since both are small angles, the following equation applies:

$\frac{( {L\; N} )}{({KL})} = \frac{({KL})}{2({KM})}$

transformed to:

(KM)×(LN)=½×(KL)²

Another equation, expressing the max. 1% difference in distances is:

(LN)= 1/100×(KM).

The two equations combined give:

(KM)²=50(KL)², (KM)=7.07×(KL)

As calculated in connection with FIG. 4 a, (KL)=9.4 cm, and therefore

(KM)=7.07×9.4=66 cm.

A similar calculation can be made re the differences in wavelengths fromroller (15) to roller (7).

In FIG. 5, the rotatable corrugated parts of the grooved banana rollerconsist of many rings (25) which through ball bearings (26) are fixed tothe circularly bent shaft (23). Shimrings (24) and (24 a) secure theproper performance of the ball rings.

The construction of a smooth banana roller, as shown in FIG. 6, issimilar to that shown in FIG. 5, except that the rings (25) do not havecorrugated shape, and that arm (27) connected to the framework of themachine, support bent shaft (23) at its middle. Without such support,the tension in the film may distort the plane determined by the curvedaxis of the shaft. In order to simplify the drawing, the support arm(27) is shown parallel with this plane, i.e., the plane of the paper,but more practically it should be arranged obliquely with respect tothis plane to best possible counteract the film tension.

Corresponding to the support arm (27) there is a “half-ring” (28) whicheither is an extension of this arm, or is fixed to shaft (23). The filmslides over this half-ring, and the frictional heat is removed by meansof cooling water pumped through a channel in the arm (not shown).

In FIG. 7, the stand up convolutions (101) are gradually transformed tolayflat pleats (103) all laying to the same side. One positiontherebetween (102) is shown. The pleats are laid flat like this all tothe same side, on the smooth banana roller (15) and the smooth straightroller (7), see FIG. 2. The device to perform this operation may be acomb-like array of gradually twisting thin metal plates. This means thatat their upstream end they are generally perpendicular to the axes ofthe smooth roller and, towards their downstream end, changing theirangle to be parallel to this axis. Without any guiding deviceimmediately upstream of each of these two smooth rollers, these rollerswill tend to randomize the pleating. To some extent this randomizationcan also be counteracted by a simple comb or an array of freely rotatingdiscs, but such a simple arrangement will not turn the pleats over tothe same side.

In FIG. 8, the grooved banana roller is substituted by many short,straight grooved rollers (29), each supported at its ends by a ballbearing (30) and (31). Each pair of adjacent ball bearings is encased ina housing (32), the housing being fixed through an arm (33) to theframework of the machine or to means for opening and closing theintermeshing between the rollers.

FIG. 9 is identical with FIG. 8 except that, like in FIG. 7, there arenonrotatable water-cooled “half-rings” (34) over which the film slides.

In a substitute construction of the first banana roller shown in FIGS. 2a and b, the fixed shaft on which the short segments can idle, connectedto the shaft through ball bearings. As mentioned, this can be a suitablealternative to the first banana roller. The advantage of building up acrown-shaped roller of many short, independently moving segments is thateach segment can follow the velocity of the film, almost without anysliding over the roller surface shows a part of a crown-shaped rollercomposed of many segments which can be rotated independent of eachother.

Example

A tubular film of 100 micron thickness is extruded from the followingcomposition: Middle layer, 70% of total: 100% HMWHDPE.

Internal surface layer, 10% of total: LLDPE of m.f.i.=1.

External surface layer, 20% of total: 60% metallocene PE+40% LLDPE;m.f.i.=1.

Width of the lay-flat film: 54 cm. The lay-flat film is pleated andstretched at 15° C. in the apparatus shown in FIGS. 2 a and b, with themodifications appearing from the following:

The radius of banana roller (15) is 1.00 m and the radius of bananaroller (14) is 1.50 m, still giving the degree of pleating 1.5:1. Thestretching takes place in two steps only. The grooved roller pair (16)is omitted. The rollers of pair (18) have radius 1.06 m and those ofpair (17) 1.15 m. The pitch of the driven grooved rollers (7) is 15 mm,and from this the pitch of the other grooved rollers is calculatedsimilarly to the calculations in connection with FIG. 4 a.

The temperature of the stretching rollers are maintained at 15° C. bymeans of circulating water. In the oven (114) the film is treated at 70°C. under a low tension, controlled by roller (118) with connecteddevices.

The stretching ratio measured as the ratio between the velocity of thelast and the first set of stretching rollers is 2.8:1, and the finalstretching ratio after the relaxation is 2.8:1.

The lay-flat tubular film then is helically cut to produce a single filmin which the main direction of orientation forms an angle of 45° to themachine direction, and is continuously laminated to a similar filmbetween nip rollers at 70°, whereby the metallocene containing layersare acting as lamination layers.

We claim:
 1. An apparatus for longitudinal orientation of athermoplastic film (4) comprising, in sequence in the machine direction:i) a width-reducing station comprising at least one pair of intermeshingpleating rollers (16) comprising intermeshing grooved rollers orintermeshing sets of discs for application of regular longitudinallyextending pleats across a width of the film; and ii) a longitudinalstretching station for stretching the film longitudinally in the solidstate, comprising one or more spaced pairs of stretching rollers (9, 10)or bars, the spacing between each pair being relatively short, where thewidth-reducing station further comprises an upstream roller (14) orroller assembly and a downstream roller (15) or roller assembly with awidth reduction zone located therebetween, the upstream and downstreamroller or roller assembly having varying directions for the respectiveaxis of rotation, the variation being from 90° with the machinedirection at a center of the film, gradually varying on both sides ofthe center towards edges of the film, so that the film is directedthrough the width reducing station while the edges of the film convergetowards the center.
 2. The apparatus according to claim 1, furthercomprising: a distance between the upstream roller (14) or rollerassembly and downstream roller (15) or roller assembly is no longer than3 times the original film width.
 3. The apparatus according to claim 1,wherein the downstream roller (15) is a banana roller with its convexside pointing towards the upstream roller or roller assembly.
 4. Theapparatus according to claim 1, wherein the downstream roller assemblycomprises many short rollers (29) mounted individually and togetherforming part of the circumference of a polygon for the desired patternof pleating.
 5. The apparatus according to claim 1, wherein the film isdirected from the downstream roller (15) towards the longitudinalstretching station in a direction deviating no more than 10% from theperpendicular to the direction it moves through the width-reducingstation.
 6. The apparatus according to claim 1, wherein the upstreamroller (14) is a banana roller with its concave side pointing towardsthe downstream roller or roller assembly.
 7. The apparatus according toclaim 6, wherein the banana roller forms an arc, the tangents of whichare perpendicular to the film tension created by the downstream rolleror roller assembly.
 8. The apparatus according to claim 1, furthercomprising: a film supply means (6) upstream of the width-reducingstation in which the film is directed towards the upstream roller (14)or roller assembly in a direction deviating no more than 10° from beingperpendicular to its movement within the width-reducing zone.
 9. Theapparatus according to claim 1, wherein the upstream roller (14) orroller assembly is a crown-shaped roller or an assembly of short rollerswhich together form a crown-shape on a straight shaft, the short rollersbeing connected with a common shaft through bearings to be able torotate independently of each other.
 10. The apparatus according to claim1, wherein the width-reducing station further comprises at least onepair of rollers comprising mutually intermeshing discs (22) locatedbetween the upstream and downstream rollers or roller assemblies, in theform of pairs of grooved banana rollers.
 11. The apparatus according toclaim 10, wherein the grooved banana roller consists of discs ofdifferent external diameters in alternating succession or of shortroller segments supplied with grooves, the discs or roller segmentsbeing mounted on a bent shaft.
 12. The apparatus according to claim 10,wherein some or all of the intermeshing discs are individuallyadjustable in their intermeshing.
 13. The apparatus according to claim10, further comprising: at least one pair of mutually intermeshinggrooved banana rollers, wherein the intermeshing is variable by means ofan adjustable force acting to move the two rollers together.
 14. Theapparatus according to claim 1, wherein the width-reducing stationfurther comprises a set of narrow conveyor belts following and guidingthe film through at least a part of the zone between the upstream (14)and downstream (15) rollers or roller assemblies, the two sets of narrowbelts gradually intermeshing more and more with each other during theconverging advancement of the film in this zone.
 15. The apparatusaccording to claim 1, wherein the width-reducing station furthercomprises several pairs of intermeshing pleating rollers (16, 17, 18),the pitch of the grooves between successive pairs decreasing.
 16. Theapparatus according to claim 1, wherein downstream of the pleatingrollers and immediately upstream (8) of at least one smooth roller (7)which is upstream of the longitudinal stretching station there areprovided pleat guiding means adapted to fold the pleats (101) all in onedirection (103), preferably comprising screwed tracks.
 17. An apparatusfor processing of a film comprising a grooved banana roller.
 18. Theapparatus according to claim 17, further comprising a pair of mutuallyintermeshing grooved banana rollers.
 19. The apparatus according toclaim 18, wherein the intermeshing is variable by means of an adjustableforce acting to move the two rollers together.
 20. The apparatusaccording to claim 17, wherein the banana roller comprises discs ofdifferent external diameters in alternating succession or of shortroller segments supplied with grooves, whereby the discs or rollersegments are mounted on a bent shaft through bearings, or they may inthemselves act as bearings.